It is known to make electrodes for discharge lamps which are formed by a core pin or the like and one or more wire windings or wraps about the core, in which the wire wraps or windings may have a single layer or a plurality of layers above each other. The tip of the core pin extends beyond this electrode head formed by the wrapped wire, in order to provide a discharge point for the discharge arc. U.S. Pat. No. 4,396,857, the disclosure of which is hereby incorporated by reference, has an electrode structure of this type.
It has been found that high-pressure discharge lamps with metal halides which contain specific fills, so that the color temperature will be between about 2700 to 3400K, cannot use such an electrode construction. The fill for such lamps contain sodium and/or tin, so that the desired color temperature can be obtained. In operation, the fill of this lamp will result in rapid burning-off of the electrode tips. As a consequence, the arc will extend and the operating voltage as well as the re-ignition peak will increase. The burning-off of the electrode thus limits the useful life of the lamp, which extinguishes or fails when the re-ignition peak exceeds the no-load voltage.
Burn-off of the electrode tips can be reduced by constructing the electrode shafts in sturdy massive construction within the region where the arc is struck. Unfortunately, however, this solution is not suitable in lamps intended for operation directly from a power network of, for example, 50 to 60 Hz frequency, since the flicker factor is substantially increased. Use of these lamps for interior or general service illumination, for which they are particularly suitable, is visually annoying.